Part 1: The Ashes
Katniss takes stock of her new position as a rebel in District 13. She and a few others were extracted from the Quarter Quell (Katniss's second Hunger Games adventure), but Peeta wasn't one of them – he's still missing, under control of the Capitol. Katniss is still haunted by President Snow. After much angst, she decides to be the figurehead, the Mockingjay, for the rebel side. In return, she gets immunity for her friends and permission to kill Snow. Another Games victor, Beetee, makes Katniss a special weapon.
Katniss has to star in commercials for the rebel side, called propos. For one propo, she and a team that includes her old friend Gale are sent into District 2, where they see a hospital get destroyed. This inspires Katniss to call for justice. Because the rebels are gaining in strength, and because Peeta has sent out a coded message to the rebels warning them of an attack, the Capitol releases footage of Peeta being tortured.
Part 2: The Assault
Katniss is horrified by the way Peeta is being treated. The Capitol bombs District 13, just as Peeta warned it would. Citizens of District 13 retreat underground for several days. When it's all clear, Katniss makes another propo, while rebel teams go to rescue Peeta.
The rebels get Peeta and some other folks out of the Capitol, but Peeta's been badly damaged. He's been "hijacked" (as in, brainwashed) and now thinks of Katniss as the enemy. As soon as he sees her, he tries to kill her. Not-so-happy reunion indeed.
Once she recovers from Peeta's attack, Katniss needs to be part of the action. She heads back to District 2 and is part of a summit about whether to attack the other side humanely or inhumanely. Gale suggests some inhumane kinds of weapons and strategies. During the battle that follows, Katniss gets shot. While recuperating, she bonds with another victor who we met in Catching Fire, Johanna, and witnesses the wedding of victors Finnick and Annie.
After a second recovery, Katniss trains to be part of the fighting again. She, Gale, and Finnick are assigned to a special unit that will be filmed during combat and turned into more propos. Right before they go out into the field, Peeta's assigned to their unit too. Hmm. Is it just us, or does that seem like a bad idea?
Part 3: The Assassin
Katniss is afraid for her life. Though Peeta has improved a bit, he's still hijacked and hates Katniss. Her commander, Boggs, agrees that she should watch out. But Boggs is one of the first casualties. Before he dies, he gives her command of their unit.
Katniss tells the others she has a secret mission to kill President Snow, and they follow her on what turns out to be a suicide mission. One by one, their team members are sacrificed as they make their way to the heart of the Capitol. Only five of them, including Katniss, Gale, and Peeta, are left by the time they reach it. Then those three get separated.
During the ensuing battle, many innocent children die. Members of the rebels' medical team, including Prim, Katniss's sister, are also killed. Katniss is badly wounded again. On the positive side, the rebels have gained control and have captured President Snow. Katniss visits the imprisoned President Snow, who says she should blame President Coin (of the rebels) for the attack and Prim's death.
And maybe he's right. It appears the rebels staged the final attack, and the weapon Gale made (you know, the one Katniss thought was inhumane) could have been the one that killed Katniss's sister. Katniss can't forgive him.
In a meeting right before Snow's execution, President Coin says that the remaining Hunger Games victors must vote to determine whether or not the new rebel government should start up the Hunger Games again, as a way to punish the citizens of the Capitol for what they did to the citizens of the Districts. (Wait, we thought the rebels were fighting to end cruel practices like the Hunger Games?) Katniss votes in favor. In part because of Katniss's vote, the answer is yes – the Hunger Games will start again.
Then it's time for Snow's execution. Katniss has the task of shooting him with her famous bow and arrow, but at the last minute, she kills Coin instead. Snow dies anyway.
At this point, Katniss has a serious breakdown. Time passes, but she's pretty much in a fog. Later she learns that she was acquitted during her trial, and she's sent back to the ruined District 12, along with Haymitch and later Peeta. Gale disappears into another district.
Over time, Katniss is able to grieve for her sister and the many others who died. As she begins to rebuild her life, she and Peeta restore a romantic relationship and eventually have children.